There are a host of organizations that track congressional elections and offer lists of the most competitive Senate races. You can consult Real Clear Politics’ list, which is backed up by polling data, or peer into Larry Sabato’s Crystal Ball for a political scientists’ perspective. But perhaps the best indicator for which elections are most competitive comes the parties themselves.
Back in the brief window of time during which Newt Gingrich appeared to pose a threat to Mitt Romney’s candidacy, I spent a fair bit of time following him around Florida, crisscrossing suburbanized I–4, listening to Gingrich promote futuristic visions of space exploration and bemoaning the barrage of negative TV ads. Newt let things get to his head a little after his upset win in South Carolina; beyond overambitious pledges to build a moon colony by 2020, Gingrich began envisioning himself in the White House, spending more time talking about how he needed to have a Republican Congress alongside him rather than the urgent need to displace Romney.
With Rick Santorum finally out of the picture, the Romney campaign is reportedly starting its VP hunt, but there's no announcement on the immediate horizon. Recent hire Ed Gillespie will lead the search, according to Buzzfeed, and it will likely be a long process to make sure the party doesn't repeat its 2008 mistake in selecting someone ill-prepared for the national spotlight.
Liberals often complain about the Democrats’ seeming inability to message their ideas with the same consistency and verve as conservatives. It just never seems like the party has the same discipline in its talking points. Congressional Dems' messaging during the health-care reform legislation in 2009 is a case in point. Rather than taking their cues from Republicans (despite the atrocious polices it entailed, naming a bill the PATRIOT Act immediately after 9/11 was a genius tactic), Democrats went for the unmemorably named "Patient Protection and Affordable Care Act." It’s not like Democrats are clueless to such tricks—the campaign finance disclosure bill they’ve proposed after Citizens United had the fitting acronym DISCLOSE—they just didn’t bother in this instance.